1. Field of the Invention
The present invention relates to high-energy lasers. More specifically, the present invention relates to the fabrication of solid-state laser crystal and ceramic components and pre-forms having non-uniform gain profiles.
2. Description of the Related Art
High-energy lasers (HELs) are currently being evaluated for military and industrial applications. Solid-state lasers have shown considerable promise for this application. A solid-state laser typically includes a laser amplifier mounted within a laser resonator. The resonator provides the feedback necessary to build oscillation within the laser. The bulk laser active medium of the laser amplifier may be in the shape of a slab, rod or disk. When pumped, the medium provides amplification. The provision of reflective gratings at the ends of the active medium provides a resonator.
In a typical laser, an incoherent light source imparts energy to the lasing medium, which produces light in which the waves are in phase through particular electron transitions. Where the lasing medium is properly designed, this ‘coherent light’ is emitted as a beam. In certain cases, it is desirable that the emitted beam of coherent light be more intense than naturally occurs from the lasing medium. A Q-switched, pulsed laser has been developed for this purpose.
The pulsed laser contains a light controller termed a Q-switch which limits the buildup of light reflecting back and forth within the laser resonator until it reaches some selected value, at which time the growth of the internal wave rapidly increases and the threshold for laser action is reached. After releasing the built-up, light energy as a pulse, the Q-switch recovers to its prior function of restraining the light energy until the intensity is high enough for another pulse. Very high peak powers and beam energies can be achieved in this manner. Electrical, mechanical, and passive Q-switches are known in the art. The theory of operation of the passive Q-switch is described in detail in Koechner, specifically for organic dye and radiation-induced color-center saturable absorbers. See Solid State Laser Engineering, by W. Koechner, 2nd Ed., Springer-Verlag, Berlin, pp 437-442 (1988). See also U.S. Pat. No. 4,833,333, issued May 23, 1989 to Stephen C. Rand and entitled SOLID STATE LASER Q-SWITCH HAVING RADIAL COLOR CENTER DISTRIBUTION.
Commercial laser components are typically comprised of single crystals, such as Yb:YAG (ytterbium doped yttrium-aluminum-garnet) or polycrystalline transparent ceramics, such as YAG [Y3Al5O12 or Y(sub 3)Al(sub 5)O(sub 12)]. Crystal lasers, doped with an active ion, often use one or more flash lamps or laser diodes to provide ‘pump light’. The diode pump light excites the active ions in the doped crystal or ceramic gain medium to a higher energy state. This process is known as “absorption”. A “pump cavity” typically contains a uniformly doped gain medium, which may be a crystal or glass or polycrystalline element fabricated in the shape of a rod, slab, or disk; and other elements, such as a pump light reflector or relay optics. Pump light is coupled into the cavity, typically with one or more flash lamps or laser diodes, either from the side of the cavity, known as ‘side-pumping’, or the end of the cavity, known as ‘end pumping’.
The laser is created by placing the doped medium and pump cavity in the resonator. The resonator reflects photons created by spontaneous emission, i.e. those generated by the normal decay of the excited ions, and amplified by stimulated emission. For example, mirrors placed at either end of the doped medium and aligned perpendicularly to its longitudinal axis form a laser resonator. If the resonator is properly sized and a sufficient number of photons are reflected back and forth within the resonator so that the “gain” exceeds the ‘loss’, a laser oscillation will build up from spontaneous stimulated emission, i.e. ‘lasing’ will occur, producing laser light. Laser light is typically extracted from the doped medium in the pump cavity along the longitudinal axis. Pump cavities are discussed in W. Koechner, Solid-State Laser Engineering, ch. 6, 3rd edition, Springer Verlag (1992).
Efficient absorption, in which nearly all of the pump light is absorbed by the doped medium, is a primary goal of laser designers. One method of attaining efficient absorption is by using high-absorption (highly doped) laser materials. A ray of pump light going through a doped crystal one time is known as a ‘pass’. With most existing designs, a pump light ray makes only one or two passes through the doped crystal before escaping, necessitating the use of high-absorption materials to achieve efficient absorption. Absorption is governed by an exponential function. Thus, when such a crystal is side-pumped, non-uniform absorption and thus non-uniform gain often result, with the highest gain being near the edge of the lasing medium. The concentration of gain near the edge of the medium leads to problems with parasitic oscillation and amplified spontaneous emission (ASE), extraction, efficiency, and beam quality (mode control). This is particularly problematic with respect to rod shaped media.
A number of technical approaches have been implemented over the years to address the spatial beam quality, including intra-cavity apertures and specific resonator point designs for optimal performance at a specific output power. However, problems with intra-cavity apertures have been encountered including losses, active alignment and the possibility of damage, as the result of the introduction of such components into the resonator.
Custom or specific resonator point designs for optimal performance at a specific output power are problematic inasmuch as such designs are, by definition, only suitable for one power, and therefore are especially susceptible to performance degradation as the resonator or pump cavity components change.
By introducing a distributed spatial filter within the rod itself, the prior-art intra-cavity aperture can be eliminated. This approach is disclosed and claimed in the above-identified '462 application of R. W. Byren and D. Sumida. This Application details the design and fabrication of spatially-doped Q-switches and laser pump cavities, having a dopant in two different valence states. The valence densities are radially dependent and increase or decrease over the radius of the laser rod. With this technique, a desirable radial profile of the second valence state can be obtained by means of oxidation-assisted or reduction-assisted heat treatment. For example, the reduction-assisted treatment leads to conversion of the laser-active trivalent Yb3+ ions into inactive divalent Yb2+ ions due to the thermally activated mass transport of ions and oxygen vacancies. The use of the time and temperature profile teaching of the '462 application enable fabrication of laser components with a desired coordinate-dependent valence density. This, in turn, yields devices with more uniform cross-sectional gain profiles.
Unfortunately, crystal and ceramic rods having radially-dependent gain profiles in accordance with the method taught in the '462 application have created other problems due to the fact that the valence conversion effected thereby on the end faces and ends of the barrels of the rods may adversely affect the operation of the laser.
Hence, a need remains in the art for a system or method for fabricating gain media for high energy lasers with nonuniform cross-sectional gain profiles spatially contoured with respect to the exterior surfaces thereof.